The present invention relates to a method for reducing the molecular weight of a source polycarbonate having a molecular weight in the range of about 22,000 to about 68,000 using an effective amount of a tetraorganophosphonium carboxylate salt as a redistribution catalyst. The term "redistribution" is defined herein as a process for reforming a source polycarbonate having an initial weight average molecular weight to a polycarbonate having a lower weight average molecular weight.
Manufacturers of polycarbonate often produce several grades of polycarbonate falling within a molecular weight in the range of about 16,000 to about 68,000. In many instances, due to the exhaustion of one or more particular polycarbonate grades to satisfy particular marketing demands, build-up in the inventory of rejected or recycled polycarbonate, there can result excesses or shortages of polycarbonate in a particular molecular weight range.
Procedures for converting existing inventories to anticipate future needs or satisfy immediate demands in a particular polycarbonate grade, including the use of recycled or rejected material are often implemented. Polycarbonate modification can be achieved by blending one grade of polycarbonate with a different grade of polycabonate. Another procedure which can be used involves direct modification of molecular weight employing a redistribution catalyst.
There is shown in EP 0595608 (Nov. 27, 1993), a redistribution procedure for modifying organic polycarbonates, such as polymers prepared by the heterogeneous interfacial polymerization of an organic bis hydroxy compound, such as 2,2-bis(4-hydroxyphenyl)propane (bisphenol A). A wide variety of redistribution catalysts are illustrated, including a tetraalkylammonium base, such as a tetraalkylammonium hydroxide which is preferred. Another polycarbonate redistribution procedure is shown by Krabbenhoft et al., U.S. Pat. No. 5,021,521 employing a catalyst such as tetrabutylammonium tetraphenylborate.
Although various redistribution procedures are available for modifying the molecular weight of polycarbonate to satisfy marketing demands, polycarbonate manufacturers have found that the use of redistribution catalysts often can result in stability problems, or environmental concerns. It has been found for example that a tetraalkylammonium base, such as a tetraalkylammonium hydroxide can form amine residues which can lead to polycarbonate degradation. Upon the degradation of tetraphenylborate catalysts, such as tetrabutylammonium tetraphenylborate, benzene can be generated as a by-product.
It would be desireable, therefore, to be able to produce a wide variety of polycarbonates from a source polycarbonate by a redistribution procedure which did not result in the generation of deleterious or environmentally unattractive residues.